1. Field of the Invention
The present invention relates to methods of producing carbon-containing component parts, and more particularly, to methods of protecting carbon-containing component parts of metallurgical units from degradation by oxidation and may be used in ferrous and non-ferrous metallurgy.
The term "metallurgical units" as used herein and hereinafter refers to electrolytic cells for the electrolytic production of non-ferrous metals such as magnesium or alumunium, to electric melting furnaces such as ore melting furnaces for the production of titanium slags and to blast furnaces, and the term "carbon-containing component parts" refers to graphite, graphitized and carbon electrodes of electrolytic cells, electric melting furnaces as well as to carbon-containing lining blocks for blast furnaces.
2. Description of the Prior Art
It is well known that intensive oxidation of carbon-containing component parts of metallurgical units by the oxygen of the air at elevated operating temperatures of the order of hundreds of degrees C is the main factor determining the losses of the mass of said component parts and their deformation due to localized burning of a carbon-containing material.
For example, in electric arc melting furnaces, the thermooxidizing destruction of graphitized electrodes away from an arc zone is quantitatively comparable to the electroerosive destruction and air burning of the same electrodes within the electric arc zone.
The thermooxidizing destruction of the graphitized anode portions disposed above the melt in electrolytic cells adapted to produce magnesium is so intense that it is necessary to replace the anodes in the course of the campaign of the electrolytic cell. This proves that the problem of increasing the resistance of the carbon-containing component parts to oxidation is very important, since consumption of said component parts largely affects the economy of processes for the production of non-ferrous and ferrous metals in the above metallurgical units.
Preventing the carbon-containing component parts of the metallurgical units from being exposed to the oxygen of the air by sealing the working space and further evacuating it or filling it with inert gases is advisable only for small metallurgical units adapted for the production of exceptionally pure metals and alloys in small quantities.
Therefore, efforts in searching for ways and means of protecting the carbon-containing component parts of large metallurgical units against the thermooxidizing destruction are mainly directed to a decrease in the rate of diffusion of the oxygen of the air into depth layers of the carbon-containing material or to preventing the surface and surface layers of the carbon-containing component parts from being acted upon by oxygen.
The decrease in the rate of diffusion of oxygen in the depth layers of the carbon-containing material can be attained by the production of the carbon-containing component parts from high density graphite. However, reserves of such graphite are limited. The same object can be attained by the production of dense carbon-containing component parts having porosity of 10 to 15% by pressure shaping, such as compression or extrusion. However, application of such methods involves an increase in the cost of the dense carbon-containing component parts 1.3 to 1.5 times as compared to the cost of the carbon-containing component parts having a porosity of 23 to 25%. It is to be noted that the coefficient of thermal expansion of dense carbon-containing component parts is higher than that of high-porous carbon-containing component parts. Therefore, the dense component parts turn out to be less stable under sharp temperature drop occurring in the working space of the metallurgical unit and are liable to cracking and cleavage fractures.
These disadvantages can be overcome to a large extent when employing methods providing a protective coating on the carbon-containing component parts, as is disclosed in U.S. Pat. Nos.: 3,060,115; 3,236,753; and 3,303,119. In accordance with these patents, the upper portions of a carbon electrode are coated with a thick layer of cryolite or alumina to form a protective coating. As practice has shown, these methods when used in protecting electrodes of electrolytic cell for the production of magnesium also suffer from a number of disadvantages. In particular, air or an aggressive medium under high-temperature conditions diffuse through the protective coating, fill up the pores of the electrode and cause oxidation. The resulting carbon oxide and dioxide volatilize through the coating, the bond between the grains of carbon gets broken and they fall off under the protective coating. This leads to higher electrical resistance of the electrode, increases voltage, disturbs the normal course of the electrolytic process and couses an excessive consumption of electric power. In addition, the electrodes must be replaced from time to time, since their service life is considerably shorter than the operating period of the electrolytic cell. When replacing a waste electrode with a new one, the protective coating of the latter may be torn, which brings about rapid destruction of this new electrode.
Most widely spread methods of protecting carbon-containing electrodes are those, wherein the electrodes are impregnated with various phosphorus-containing compositions as is disclosed in U.S. Pat. No. 3,029,167.
Such method of protection used for the first time as early as 1929 in Germany by the firm I. G. Farbenindustri has been modified but slightly. According to German Pat. No. 580190, the above method comprises impregnating the electrode with orthophosphoric acid. The orthophosphoric acid fills up the pores throughout the whole volume of the electrode and, when heated under operating conditions, changes to pyrophosphoric and metaphosphoric acids, thickens and turns to a polymer. The polymer is a glassy mass intimately filling the pores of the electrode and protecting the electrode from oxidation. Service life of the electrodes treated as described above is 3 to 9 months and small mechanical damages practically do not make it shorter. Other characteristics which distinguish the method consists in easy fabrication of the electrodes and their comparatively low cost due to utilization of a cheap carbon-containing material having a porosity of 20 to 25%.
However, despite the obvious advantages, this method suffers from a serious drawback limiting the service life of the carbon-containing component part. In particular, at a temperature of 350.degree. C. the metaphosphoric acid starts to intensively evolve phosphoric anhydride P.sub.2 O.sub.5 which volatilizes. This phenomenon brings about a severe deterioration in the efficiency of carbon protection at a temperature of 400.degree. C. and at the same time considerably limits the field of application of the method rendering it unsuitable for the protection of carbon-containing lining blocks of blast furnaces, electrodes of ore melting furnaces and other high-temperature metallurgical units.